Project Summary Major depressive disorder (MDD) affects ~4% of the worldwide population, and these numbers are continuously increasing. Moreover, MDD disproportionately affects women, who are twice as likely as men to be diagnosed with the disease. Thus, uncovering the molecular, cellular, and circuit-level mechanisms underlying susceptibility and resilience to depression is critical. Recent work demonstrates that ventral hippocampal (vHPC) projections to nucleus accumbens (NAc) regulate mood and susceptibility to stress. Our group has shown that the transcription factor ?FosB is crucial for hippocampal cell morphology and learning, and several lines of evidence implicate hippocampal ?FosB in depressive-like behaviors (e.g., FosB knockout mice have hippocampal malformations and depressive behaviors, and current antidepressants induce ?FosB in vHPC). However, to date there is no study characterizing the role of hippocampal ?FosB in stress resilience and depression. To this end, we have produced a novel dual-virus CRISPR-Cas9 system allowing knockout of FosB gene expression specifically in vHPC cells projecting to the NAc. In combination with our many well-vetted molecular, physiological, and behavioral tools and models, this will allow us to address the central hypothesis that ?FosB expression in vHPC neurons projecting to NAc mediates cell function and resilience to social stress in male and female mice. To test this hypothesis, we propose two Specific Aims. In Aim I, we will first characterize the epigenetic mechanisms of induction of the FosB gene in hippocampus by chronic stress, determine its role in resilience to stress in both males and females, and uncover its transcriptional targets in vHPC. Indeed, our preliminary data show that inhibition of ?FosB in vHPC induces susceptibility to depression-like symptoms. In Aim II, we will examine the stress induction and cellular effects of ?FosB in vHPC neurons with projections to various brain regions, including NAc, then use our novel dual-virus system to knock down FosB gene expression specifically in vHPC cells projecting to the NAc and observe the behavioral results on stress responses. Here, too, we have strong preliminary data suggesting that ?FosB regulates the excitability of vHPC neurons, and that ?FosB expression in the vHPC- NAc projection cells regulates susceptibility to depression. Together, the proposed studies will elucidate the critical role of ?FosB in stress-induced changes in hippocampal control of limbic circuitry of both males and females, as well as identify novel ?FosB targets that could serve as potential points for sex-specific therapeutic intervention in depression or PTSD.